Abstract: In the sea ice numerical simulation of the Bohai Sea,under the same meteorological and oceanographic conditions,the ice thickness always trends to a certain value,which can be called diagnostic thickness,although the initial thickness is different.Through the analysis of ice growth mechanism,the existence of diagnostic thickness is discussed.The ice temperature and the heat flux at ice-cover surface,the transmitted solar radiation and the heat flux at ice-cover bottom under different ice thicknesses are contrasted.The dynamic process and the approach time when the sea ice approaches the diagnostic thickness are discussed.Under different air temperatures,wind speeds,relative humidities and oceanic heat fluxes,the diagnostic thicknesses are calculated respectively.The relationship between the actual ice thickness and the diagnostic thickness in the winter of 1997/1998 of the Liaodong Gulf are analyzed.The study of the sea ice diagnostic thickness will be useful to the sea ice numerical simulation and the de-termination of the sea ice design parameters in the Bohiai Sea.